Electrodeposition paint has been put to wide usages including coating bodies and other parts of automobiles, and development of electrodeposition paint compositions having various characteristics, for example, those excelling in corrosion resistance, electrocoating ability, of rustproof steel sheet, film-forming ability, stability and the like, has been attempted.
Namely, organic solvents having boiling points not higher than about 120° C. [e.g., methyl isobutyl ketone (116° C.), methyl ethyl ketone (80° C.) and the like]; organic solvents having boiling points ranging 120-200° C. [e.g., ethylene glycol monobutyl ether (171° C.), propylene glycol monomethyl ether (121° C.) and the like]; or low molecular weight soft resins of molecular weights not higher than 4,000 (e.g., xylene resin, polypropylene glycol and the like) are incorporated in electrodeposition paint compositions, for maintaining the latter's film-forming ability over prolonged periods. Recently, however, use of such organic solvents is restricted according to low VOC (Volatile Organic Compounds) regulations and HAPs (Hazardous Air Polutants) regulations, in consideration for preservation of good environments.
On the other hand, when the content of volatile organic compounds in electrodeposition paint is reduced, film-forming ability (film thickness retention) gradually deteriorates with time, rendering it difficult to apply the coating of a prescribed thickness. This also introduces a number of problems including impaired appearance and occurrence of pinholes (occasionally referred to as “gas” pinholes) in alloyed molten zinc-plated steel sheet (zinc-iron alloy-plated rustproof steel sheet) which is frequently used as outer panels of automobile bodies (e.g., doors, fenders and the like) to induce deficient coating.
It has been also practiced for the purpose of reducing the volatile organic compound content, to remove the content to volatile component such as an organic solvent in electrodeposition paint by reduced pressure distillation or the like means, until the content becomes not more than 1 wt %. It gives rise to problems such as coating to no thinner than 20 μm in terms of cured coating film thickness becomes difficult, or melt fusing ability of the coating film is deteriorated, which leads to easier occurrence of pinholes.
U.S. Pat. No. 4,891,111 discloses a cationic electrodepositon paint which contains alkylated polyether having at least three ethereal oxygen atoms and 1-4 saturated hydrocarbon groups between the ethereal oxygen atoms per molecule but having no hydroxyl group. This cationic electrodeposition paint has a low VOC content and its coating film exhibits good appearance and corrosion resistance, but has problems in electrocoating ability for rustproof steel sheet, film-forming ability and paint stability. The paint, therefore, is not fully satisfactory.
U.S. Pat. No. 6,342,546 discloses a cationic electrodeposition paint which contains alkylene polyether polyol such as polymethylene glycol, polyethylene glycol, polypropylene glycol, bolybutylene glycol and the like; or polyether polyol such as aromatic ring-containing polyether polyol obtained with use of bisphenol alone or in combination with glycol. This cationic electrodeposition paint has a low VOC content and favorable film-forming property, and gives coating film excellent in electrocoating ability of rustproof steel sheet (i.e., pinholing resistant property in alloyed molten zinc-plated steel sheet) and corrosion resistance. On the other hand, when a large amount of the polyether polyol is added to the paint, problems arise such as deterioration in the corrosion resistance (the deterioration is drastic under severe corrosive environments) or paint stability and reduction in sealer adherability onto the electrocoated film.